The invention relates to a speed indication signal generator circuit which generates a speed command signal for controlling an engine on an automative vehicle to a desired actual speed; more particularly the invention relates to a speed indication signal generator circuit which generates an idling speed command signal for controlling an engine on an automative vehicle to an desired idling actual speed in response to the temperature of the cooling water in the engine.
Hitherto, a speed indication signal generator circuit was employed for generating a target speed signal for controlling the actual rotational speed of the engine on the automative vehicle, more particularly, for generating a target idling speed signal for controlling the actual idling rotational speed of the engine to a predetermined one in response to the temperature of the cooling water in the engine. Generally speaking, the generator circuit comprises a thermistor which detects the temperature of the cooling water in the engine. The resistance of the thermistor changes in response to the change of the temperature of the cooling water, whereby a voltage which corresponds to the temperature of the cooling water is obtained across the thermistor. Thus a speed indication signal which represents a desirable engine speed corresponding to the temperature of the cooling water is generated in the generator circuit on the basis of the voltage across the thermistor. However, the resistance of the thermistor changes exponentially with respect to the change of the temperature of the cooling water. Therefore the variation range of the voltage across the thermistor is very wide as compared with the normal variation range of the temperature of the cooling water. Thus the engine is not suitably controlled with the voltage across the thermistor, because the voltage at some temperatures of the cooling water represents an unreal engine speed. Also the voltage across the thermistor represents an abnormal engine speed when the thermistor is disconnected or short-circuited, which causes an abnormal rise or fall of the engine speed.